A farewell to airborne astronomy
Niamh Shaw takes a trip on one of the final flights of the plane carrying the SOFIA observatory
The time is 5:30pm on 8 September 2022 and I’m in the mission briefing room at NASA’s Armstrong Flight Research Center in Palmdale, California. I’m about to board a very special aircraft: the Boeing 747 SOFIA aeroplane, carrying a 2.5-metre infrared telescope operated by NASA and DLR, Germany’s space agency. It is one of SOFIA’s last flights; by the time you read this the programme will have ended.
Inside, the cabin is buzzing with activity. It’s extremely loud, so headsets are essential for communication. I tune in to the flight deck comms channel and hear pilots Spike and Bill, and flight engineer Rick. Spike’s the joker and the three enjoy a few laughs together as they wait for clearance. It’s Spike and Bill’s final SOFIA flight. They’ve been flying together for 24 years, having begun their professional relationship in the US Air Force.
The Stratospheric Observatory for Infrared Astronomy (SOFIA) was a long time coming, with development of the project beginning back in 1996. SOFIA was originally a commercial Boeing 747SP acquired by Pan Am airlines in 1977, and the aircraft still carries the name ‘Clipper Lindbergh’ in honour of aviator Charles Lindbergh. Extensive modifications were made when it was acquired by NASA in 2007, and in 2009 its telescope cavity doors were opened in full flight for the first time. In May 2010, SOFIA completed its first in-flight tracking of astronomical targets – the planet Saturn and the stars Beta Leonis and Beta Orionis – and in July 2013 it made its first deployment to New Zealand for observations of the southern winter skies.
Throughout the programme, SOFIA’s 10-hour flights have taken it as high as 45,000 feet, soaring above Earth’s distortive atmosphere and giving it an important advantage over ground-based telescopes. From that height, its infrared view penetrated cosmic clouds of dust and gas, enabling astronomers to study the lifecycle of stars, distant nebulae and galaxies and supermassive black holes residing in galactic centres. It also observed closer to home, analysing the planets, comets and asteroids in our own Solar System.
My journey with SOFIA began a few months previous, when I heard that the NASA programme had just one last deployment to New Zealand to complete. I headed to Christchurch in late July 2022 with the intention of joining operations on the ground, working out of the US Antarctic Program building. Unfortunately, severe weather in July had damaged the aircraft, and it wasn’t able to finish its Southern Hemisphere deployment. In late August, SOFIA returned to California to carry out the remainder of its mission.
High-flying science
While in New Zealand, I met Margaret Meixner, the director of science operations. “The southern deployment became an essential part of the SOFIA programme,” she told me, “because there are objects that can only be seen from the Southern Hemisphere; objects like the Galactic centre, which are incredibly important to observe.”
Mike Toberman, NASA’s SOFIA project manager, was on every New Zealand deployment. “We started in 2013,” he says, “and it was to be a short mission. But when the scientists got off the plane, they were ecstatic and said it was like observing from space. We kept extending the duration of the appointments, from six weeks to two months.”
Equipped for success
There were many bodies involved in SOFIA, with the main actors being NASA and DLR. The 2.5-metre onboard telescope was designed and maintained by the German SOFIA Institute in Stuttgart. This vital instrument observed at mid- and far-infrared wavelengths, which enabled astronomers to see regions of the cosmos that would otherwise be invisible, such as hydride molecules, some of the first molecules to form in the Universe.
“One of the science highlights of SOFIA, the biggest, probably most noted to the public, is the discovery of water on the sunlit surface of the Moon,” Meixner told me.
“Another highlight has been observing hydrides from our instrument GREAT [see Infrared instruments, below]. Hydrides possess bright rotational transitions in the far infrared. Measuring them is critical to understanding and really pinning down the processes of astrochemistry.”
SOFIA’s scientific achievements would be the envy of any ground-based observatory. It has mapped magnetic fields in objects like the Whirlpool Galaxy and supernova remnant Cassiopeia A. It captured an infrared view of temperature changes on Jupiter and studied the formation of planetary systems around distant stars. It discovered how powerful stellar winds are hindering star formation in the Orion Nebula, revealed that magnetic fields are feeding material into the supermassive black hole in galaxy Cygnus A, and detected evidence of a recent exoplanet collision in a double-star system just 300 lightyears away. In 2011, SOFIA flew 2,900km from its Californian base, over the Pacific Ocean to observe Pluto occulting a distant star, providing information about the pressure, density and temperature of the dwarf planet’s atmosphere. That’s something ground-based observatories could only dream of.
Up in the air
On board my Palmdale flight, I notice how busy the plane is at each of the console areas as scientists make preparations for the collection of data by the FORCAST instrument, SOFIA’s infrared camera and spectrograph. Callie Crowder, who is managing the telescope, prepares for the telescope door opening, the first big event of the night. Over the course of the flight we will observe edge-on galaxies, the sunlit side of the Moon and the galactic mid-plane. It’s a relentless schedule focused on maximising observation time.
I join the pilots in the cockpit upstairs and notice the mood is a lot more relaxed here. I ask Spike how he feels to be on his last-ever SOFIA flight. He says it’s a privilege to have been a part of the programme, and that the deployments to Christchurch were a particular highlight. “It felt as if the whole of Christchurch knew whenever SOFIA came to town. People were so welcoming. I was very proud of my job when on deployment there.”
He’s right. In Christchurch, whenever I mentioned the reason behind my visits to any of the locals, they perked up and shared their own SOFIA stories. But this was contrasted with disappointment that the current deployment would be SOFIA’s last. In early August 2022, as I left Christchurch on my way to California to catch my SOFIA flight, two officers at passport control took me to one side. I shouldn’t have worried, because they had just discovered the purpose of my visit and wanted to speak more about SOFIA. There had been a TV news special the night before, highlighting the mission’s cancellation. They asked if there was anything they could do to show support and wanted me to thank NASA and DLR for their visits over the years.
The end of SOFIA was tough news for the science team. NASA’s astrophysics division decadal survey in November 2021 concluded that SOFIA’s science productivity did not justify its $85 million a year operating costs. “We argued that the survey reached its conclusion based on older information and did not take into account SOFIA’s latest discoveries”, Meixner says. “I offered to provide the steering committee with an update, but was unable to do so.”
Bernhard Schultz, SOFIA’s deputy director of science mission operations, agrees. “We put into motion measurable improvements in the last four years. The last two years were crucial, but weren’t taken into consideration.”
By now it’s 3am and some of my fellow educators have disappeared, catching an hour of rest to get them through to the 5:20am landing. I’m not sure how the science teams can stay so focused, but they just keep going.
How does Meixner feel about the future of astronomy without SOFIA’s far-infrared data? “The data we have gathered with SOFIA complements data gathered by telescopes such as the James Webb Space Telescope,” she says. “But nothing can currently replace what SOFIA has shown us. The last NASA decadal survey seemed interested in developing a far-infrared or an X-ray probe. That won’t be for 10 years though. The gap in data in the intervening years will be felt.”
A farewell flight
By 5am SOFIA’s telescope door has closed and data collection is complete. We take our seats, buckle up and I tune in to the cockpit comms. Spike chokes up as the plane touches down, and we all share a round of applause in his honour. I step off the plane and onto the runway at Palmsdale at 5:30am, when Spike and Bill appear. We invite them to join us for a group photo. “It’s the end of an era, guys,” Spike tells us, holding back tears. “End of an era.”
Just a few weeks later, on 30 September, SOFIA completed its final flight. In its eight years of operation, it flew over 920 times, generating 60 postgraduate theses, 401 research publications, hosting 1,810 unique authors and co-authors, and involving over 3,000 people from the international research sector.
I asked Meixner and Schultz how they wanted SOFIA to be remembered. “For its scientific discoveries and how it pushed the field of far-infrared astronomy,” Meixner says.
“Not just as a technological marvel, but also for its unique science,” Schultz adds. “For being unique. For being amazing.”
Infrared instruments
How SOFIA’s on-board instruments equipped the plane to observe the Universe
SOFIA housed six separate instruments that were swapped out regularly to allow the telescope to collect data at wavelengths ranging from near-, mid- to far-infrared, making it a versatile observatory.
FORCAST (Faint Object InfraRed CAmera for the SOFIA Telescope)
This instrument was in operation on my flight and that evening observed edge-on galaxies, the sunlit side of the Moon and the Galactic mid-plane. It is a dual-channel mid-infrared camera and spectrograph sensitive to the mid-infrared range of 5–40μm. But SOFIA hosted a range of other instruments, depending on what astronomers wanted to observe.
GREAT (German REceiver for Astronomy at Terahertz Frequencies)
A far-infrared high-resolution multipixel spectrometer that didn’t produce pictures of stars and galaxies, but rather extremely detailed spectra of their atoms and molecules. It was used in the first-ever detection of the helium hydride molecular ion (HeH+) in interstellar space.
HAWC+ (High-resolution Airborne Wideband Camera Plus)
A camera and imaging polarimeter that imaged in far-infrared light and was used for observing the early stages of star and planet formation, as well as producing maps of magnetic fields.
FIFI-LS (Far-Infrared Field-Imaging Line Spectrometer)
A far-infrared spectrometer able to trace the formation of massive stars and peer through cosmic dust to analyse star-forming regions.
FPI (Focal Plane Imager)
A tracking and high-speed imaging camera used as a fast-frame-rate imaging photometer in the 360–1100nm wavelength range.
EXES (Echelon-Cross-Echelle Spectrograph)
Used to split light into a spectrum, enabling astronomers to study chemicals like hydrogen, water vapour and methane from molecular clouds, planetary atmospheres and protoplanetary discs.
What next now that SOFIA is over?
We spoke to two veterans of the airborne observatory about what happens now
Steven Goldman, staff scientist at the SOFIA observatory
“There is nothing like SOFIA planned for the near future. With the closeout of SOFIA, astronomers are going to lose the capability to detect important tracers and chemical transitions for at least the next decade. We are also losing our best platform for testing and improving new infrared instruments and technology. Balloon missions like GUSTO and ASTHROS, which are currently in development, will provide new far-infrared data in years to come, but won’t fly nearly as often as SOFIA. Far-infrared astronomers will, however, still have tonnes of data to study from the last eight years of successful SOFIA flights.”
Christian Fischer, project engineer on SOFIA’s Field-Imaging Far-Infrared Line Spectrometer
“Our focus now shifts from data collection to ensuring that the knowledge we learned about observing far-infrared from the stratosphere is preserved. We need to get all the data from SOFIA in the best possible shape and make sure the astronomers have all the support they need to work with this complex information. There will be some very limited access to far-infrared skies by telescopes on stratospheric balloons, but they are not as reliable as SOFIA. For now, better and more reliable balloons or satellites are needed, but that will take some time.”
Niamh Shaw is a science writer and space communicator, and the author of Dream Big: An Irish Woman’s Space Odyssey.